Cellular communication systems continue to grow in popularity and have become an integral part of both personal and business communications. Cellular telephones and similar devices allow users to place and receive phone calls most anywhere they travel. Moreover, as cellular telephone technology is increased, so too has the functionality of cellular devices. For example, many cellular devices now incorporate Personal Digital Assistant (PDA) features such as calendars, address books, task lists, calculators, memo and writing programs, etc. These multi-function devices usually allow users to send and receive electronic mail (email) messages wirelessly and access the Internet via a cellular network and/or a wireless local area network (WLAN), for example, when the devices include appropriate circuitry for WiFi and other IEEE 802.11 WLAN access.
Many of the cellular communications use packet burst transmissions as part of a Global System for Mobile communications (GSM) system, which includes the 850 MHz, 900 MHz, 1800 MHz and 1900 MHz frequency bands. Although these mobile wireless communication devices function as a cellular telephone, as noted before, the device can also operate and incorporate Personal Digital Assistant (PDA) features and send and receive email and other messages wirelessly and across the internet via the cellular network and/or a wireless Local Area Network (LAN). This function can include access to “hot spots” as part of a WiFi network using IEEE 802.11 standards.
When such devices incorporate WiFi technology, the circuits could be considered to describe WLAN products based on IEEE 802.11 standards, using one or more Access Points (APs) as “hot spots” and various numbers of clients. An AP typically broadcasts a Service Set Identifier, “network name” (SSID), using packets called “beacons” by some skilled in the art, which are broadcast every one hundred or so milliseconds at about one Mbit\s duration in some non-limiting examples. Some of these WiFi devices operate in the 2.4 or 5.0 GHz band.
A wireless access point usually connects wireless stations to an adjacent, wired local area network, and is operative similar to an Ethernet hub. The access point can relay wireless data to other compatible wireless devices and to a single, connected local area network device, such as a Ethernet hub or switch. Wireless routers are often used to integrate a wireless access point with a Ethernet switch and Ethernet router.
In a mobile wireless communications device, if the cellular capability is integrated with WiFi capability, often two different antennas are used, for example, a main cellular antenna operative at GSM or other CDMA band and a WiFi antenna operative in the at least 2.4 GHz band, and sometimes the 5.0 GHz band, making the device not only compatible with cellular GSM communications, but also compatible with WiFi communications using IEEE 802.11 standards. The antenna designs become more challenging, however, as the size and thickness of the mobile phones become smaller to meet marketing requirements and the desires of end-use consumers. In order to implement multiple antennas in a compact environment, the antennas should be designed to reduce the coupling between the various antennas. This is necessary not only to enhance radio performance, but also reduce the cost of implementing Electromagnetic Interference (EMI) filters at harmonic frequencies. Thus, the type of antenna designs used in such devices become important to reduce the mutual coupling due to the third harmonics between a GSM or similar cellular antenna, operative, for example, at 850 MHz, and a WiFi antenna operative at 2.4 GHz. Isolating these antennas can be difficult, and different feeding techniques should be introduced to enhance isolation between the two antennas. The two systems, cellular as a Wide Area Network (WAN) and WiFi need to work simultaneously, and thus, isolation between antennas is very critical.